Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliciclastic deposits

Abstract

Although the notion of an early origin and diversification of life on Earth during the Archaean eon has received increasing support in geochemical, sedimentological and palaeontological evidence, ambiguities and controversies persist regarding the biogenicity and syngeneity of the record older than Late Archaean. Non-biological processes are known to produce morphologies similar to some microfossils, and hydrothermal fluids have the potential to produce abiotic organic compounds with depleted carbon isotope values, making it difficult to establish unambiguous traces of life. Here we report the discovery of a population of large (up to about 300 mum in diameter) carbonaceous spheroidal microstructures in Mesoarchaean shales and siltstones of the Moodies Group, South Africa, the Earth's oldest siliciclastic alluvial to tidal-estuarine deposits. These microstructures are interpreted as organic-walled microfossils on the basis of petrographic and geochemical evidence for their endogenicity and syngeneity, their carbonaceous composition, cellular morphology and ultrastructure, occurrence in populations, taphonomic features of soft wall deformation, and the geological context plausible for life, as well as a lack of abiotic explanation falsifying a biological origin. These are the oldest and largest Archaean organic-walled spheroidal microfossils reported so far. Our observations suggest that relatively large microorganisms cohabited with earlier reported benthic microbial mats in the photic zone of marginal marine siliciclastic environments 3.2 billion years ago.

Cyanobacterial evolution during the Precambrian

“…For older deposits, due to loss of taxonomic characteristics, it becomes harder to classify these microstructures. Hofmann 1976;Schopf & Walter 1983;Altermann & Schopf 1995;Javaux et al 2010;Wacey et al 2011;Sugitani et al 2013). In Fig.…”

On the Age of Eukaryotes: Evaluating Evidence from Fossils and Molecular Clocks

“…ical features, wall ultrastructure or ornamentation, or typical excystment structures unknown in prokaryotic organisms (Javaux et al 2001(Javaux et al , 2003(Javaux et al , 2004Knoll et al 2006;Strother et al 2011). Many microfossils of suggested eukaryotic origin are large (.50 mm) organic-walled structures known as acritarchs (Buick and Young 2010;Javaux et al 2010). The oldest of these are microfossils described from the 3.2-billion-year-old (Ga) Moodies group (Buick and Young 2010;Javaux et al 2010), far older than most acritarchs described to date (Javaux 2007).…”

“…Many microfossils of suggested eukaryotic origin are large (.50 mm) organic-walled structures known as acritarchs (Buick and Young 2010;Javaux et al 2010). The oldest of these are microfossils described from the 3.2-billion-year-old (Ga) Moodies group (Buick and Young 2010;Javaux et al 2010), far older than most acritarchs described to date (Javaux 2007). Javaux and colleagues were very cautious in their interpretation and argue that a prokaryote origin could not be definitively excluded.…”

La vida temprana en la Tierra y los primeros ecosistemas terrestres

“…Los fósiles tipo eucarionte más antiguos (acritarcos, Buick, 2010), que quizás requerían oxígeno para maximizar sus capacidades energéticas y metabólicas, tienen ~3200 Ma de edad y también estuvieron presentes en ambientes estuarinos (Javaux et al, 2010). A pesar de desconocer su verdadera identidad, la presencia de microfósiles de gran tamaño (> 150 µm de diámetro celular) indica que la vida se diversificó y alcanzó una presencia global relativamente rápido (Kandler, 1994;Altermann y Schopf, 1995;Ueno et al, 2006;Blank, 2009;David y Alm, 2011), y ocupó una amplia variedad de nichos ecológicos hacia el Paleoarcheano, incluso en lugares gravemente perturbados por impactos de asteroides (ver Walsh, 1992 y referencias incluidas).…”